Some important aspects of fragment angular momentum in medium energy fission of U-238

Independent isomeric yield ratios of Te-131, Te-133 and I-134 have been det ermined at five different energies in the range of 25-44 MeV alpha particle induced fission of U-238 using radiochemical and gamma spectrometric techn iques, From the independent isomeric yield ratios, fragment angular momenta (J(rms)) have been deduced using a statistical model analysis. The J(rms) were also calculated theoretically based on thermal equilibration of variou s collective modes after considering the occurrence of multichance fission. These data and the literature data for various fragments in the mass regio n 126-136 in U-238(alpha,f), U-238(p,f) and U-238(gamma,f) show the followi ng important features: (i) Both the entrance channel excitation energy and input angular momentum affect the fragment angular momentum in the exit cha nnel, (ii) There are two groups of fission products from the point of view of change of fragment angular momentum with increase in excitation energy a nd input angular momentum. (iii) Fragment angular momentum depends on nucle ar structure effect such as shell closure proximity and odd-even effect. (i v) The fragment angular momentum calculated theoretically based on statisti cal equilibration of various collective modes are in good agreement with th e experimental values indicating the validity of such an assumption. (C) 19 99 Elsevier Science B.V.